This invention relates to materials testing machines, particularly to such machines which are used to measure engineering properties, and most particularly to machines for measuring uniaxial and axisymmetric triaxial loading configurations.
Materials which have isotropic material properties, and those which have transversely isotropic properties, are often tested to quantify those engineering properties using a right circular cylindrical specimen loaded with a xe2x80x9ctriaxialxe2x80x9d (axisymmetric) pressure cell. This type of testing is quite useful, in particular for geotechnical materials such as solid rock, soils, stabilized soils, aggregates, asphalt concrete and portland cement concrete. The concepts are applicable to a range of other materials as well, including plastics, composites, and any materials which do not have a homogeneous structure at the scale of the test specimen dimensions or which rely on external boundary conditions to retain their shape. In order to perform tests on materials such as pavement materials, one must do more than hydrostatic or proportional loading such as that suggested in U.S. Pat. Nos. 4,615,221 and 5,493,898 in terms of both loading and instrumentation.
U.S. Pat. No. 4,579,003 to Riley illustrates a useful device which combines triaxial loading with the capability to introduce direct shear to the specimen. However, in composite materials such as asphalt, the maximum size and size distribution of the aggregate strongly affect shear measurements because of their interaction with the specimen geometry, and many shear devices generate stress fields during loading that are functions of the material properties of the specimen which are the subject of the testing, and therefore cannot directly produce accurate measurements of those properties. While xe2x80x9cconfinedxe2x80x9d (i.e. pressurized) and xe2x80x9cunconfinedxe2x80x9d (i.e. unpressurized) tension and compression tests e.g. those tests in which the stress applied along the direction of the axis of the cylinder is greater than the all around confining pressure, a condition which results in the engineering terminology describing the axial stress being the xe2x80x9cmajor principalxe2x80x9d stress, the cyclic portion of which is also termed xe2x80x9cdeviatoricxe2x80x9d, and the radially inward stress resulting from the simultaneous application of confining pressure being termed the xe2x80x9cminor principalxe2x80x9d stress, are the most common types of tests conducted in the axisymmetric triaxial configuration, it is obvious that it is difficult, if not impossible, to conduct tension tests on materials with little or no cohesion.
In the compression test, the stress is applied toward the mid-height of the specimen, but in the tension test, the stress is applied away from the mid-height which requires some technique for attaching the loading system to the ends of the specimens and such attachment is virtually impossible for many soils and aggregates. For these materials, which include many soils and aggregates, another type of test in which the major principal stress direction is changed from being applied along the axial direction, e.g. vertical in the usual orientation, to the horizontal direction, e.g. radially inward toward the center of the specimen under test, is useful. This test is referred to as an xe2x80x9cextensionxe2x80x9d test in engineering terms and is conducted with the major principal stress direction radially inward (or horizontal in the usual configuration) and the minor principal stress direction is applied in the compression direction along the axis of the specimen. The extension test often yields engineering property data which might help understand tension behavior of the material without actually conducting a tension test, and anisotropic behavior without incurring the complexity of true three-dimensional testing on a prismatic specimen.
The instrumentation in the standard geotechnical triaxial cell has been a persistent problem in the prior art. While useful for rock specimens, instrumentation solutions such as that given in U.S. Pat. No. 4,587,739 do not work for low cohesion materials such as soils, road base materials and hot asphalt concrete in part because the devices often have high localized stress fields at the specimen contact points in order to support the mounting system. Ultrasonic testing systems such as that given in U.S. Pat. No. 5,741,971 suffer from difficult analysis procedures required to accurately quantify the properties of particulate materials, and the extrapolation of the results from the test""s high frequencies down to relevant frequencies for time-dependent and stress-dependent pavement materials is sometimes ineffective.
Some cohesive and engineered geotechnical materials are also tested using beam flexure and indirect tension loading (e.g. Standard Test Method for Indirect Tension Test for Resilient Modulus of Bituminous Mixtures by ASTM in ASTM D4123 (Apr. 30, 1982), Resistance of Compacted Bituminous Mixture to Moisture Induced Damage by AASHTO in AASHTO T283 (1989), and Resistance to Plastic Flow of Bituminous Mixtures Using Marshall Apparatus by AASHTO in AASHTO T245 (1994)), both of which general categories of test are usually performed without confining pressure.
There are two major categories of axisymmetric triaxial pressure cells. In the first type of triaxial cell (e.g. Standard Test Method for Determining the Resilient Modulus of Soils and Aggregate Materials by AASHTO in AASHTO TP46 (T294) (1994), Resilient Modulus of Subgrade Soils and Untreated Base/Subbase Materials by AASHTO in AASHTO T292 (1991)), the specimen is capped, placed in a rubber membrane, and is surrounded on all sides, top, and bottom by a confining medium inside the cell (usually air, water, or oil, the fluid obviously being selected based upon the necessary thermal, mechanical, and electrical conductivity requirements of the particular application). This configuration is termed the xe2x80x9cstandard geotechnicalxe2x80x9d configuration throughout the instant disclosure. When pressure is applied all around the specimen through the confining medium alone, a hydrostatic stress state is said to exist.
In order to evaluate material properties at a constant confining pressure, the stress along the axis of the specimen is usually changed dynamically (i.e. cyclically) through a sealed linear bearing by an actuator shaft reacting against a load frame external to the triaxial cell. This dynamic action is usually referred to as a deviatoric stress and the direction of application is usually the major principal stress direction. In some systems, the confining pressure may also be changed dynamically. Usually, the specimen deformation instrumentation associated with this configuration is either outside the cell or referenced to some relatively rigid component of the cell such as the base plate.
1. the direction of the major dynamic deviatoric stress cannot be altered from the axial direction without sophisticated analysis and control systems which enable the axial actuator to counteract the change in the axial component of the dynamically changing hydrostatic pressure, also taking into consideration (a) the frictional effects of the pressure sealed linear bearing which is necessary if the load measuring device is located outside the triaxial cell and (b) the applicable cross sectional area of the loading shaft and platen assembly, factors (a) and (b) above being addressed to a certain extent by U.S. Pat. Nos. 4,679,441 and 5,435,187;
2. measurement devices which bear on the confining medium side of the membrane must have their measurements adjusted for the expected deformation of the membrane when subjected to a pressure change;
3. local specimen inhomogeneities (e.g. rocks with adjacent voids), normal specimen bulging in compression and normal necking in tension or extension cause inaccuracy with externally referenced instrumentation devices (i.e. the deformation along the sensitive axis of the instrumentation sensor cannot be separated from the deformation in another direction that occurs due to behavior such as bulging and necking);
4. externally referenced axial instrumentation devices (e.g. an axial measurement taken from the loading platen or actuator shaft) cannot separate specimen end effect deformations from the prevailing strain field in the middle portion of the specimen;
5. externally referenced radial instrumentation devices must incorporate sensors that are provided in pairs so that specimen translation (e.g. tilting due to ends that are not parallel) can be separated from radial strain which is internal to the specimen;
6. externally referenced radial instrumentation devices that are mounted xe2x80x9cthrough-the-wallxe2x80x9d (i.e. mounted in the wall of the pressure vessel) are susceptible to critical measurement errors because any strain in the wall of the pressure vessel during testing cannot be directly separated from the strain in the specimen;
7. heretofore, attempts to measure radial strain have used devices that are either referenced to a cell component such as the base plate or use constrictors such as springs to support the weight of the devices as well as provide the necessary tension to return the sensitive components of the sensor to proper position;
8. the use of constrictors to perform the dual functions mentioned above often results in erroneous measurements because the constrictor is often so strong that it restricts the lateral movement of the specimen under load so that a true picture of the deformation is not achieved;
9. an alternative solution to the radial strain measurement problem is to measure the vertical strain and the volume change of the fluid, but this approach is very susceptible to dissolved gas in the fluid, gas that could not be removed from hidden pockets in the cell prior to testing, and membrane deformation;
10. the process of filling and draining the triaxial cell and adjusting the instrumentation for each test is extremely tedious and time consuming.
The second type of triaxial cell is the Texas triaxial type found in Triaxial Compression Tests for Disturbed Soils and Base Materials by Texas DOT in Tex-117-E (1991), which solves some of the problems inherent in the standard geotechnical cell described above. Specifically, it solves, conceptually at least but not practically, the problem of requiring sophisticated systems to change the direction of the major deviatoric stress. This is made possible because the confinement is only applied in the radial direction and the axial load is a completely separate applied stress. It is a significantly simpler matter to conduct an extension test in this type of triaxial cell than in a standard geotechnical cell. In contrast to the standard geotechnical cell, it also has the advantage for production testing of cohesive materials that the membrane is part of the cell, not a consumable part of the specimen assembly requiring a tedious installation process prior to placing the assembly inside the cell.
However, disadvantages of the Texas triaxial type of cell include:
1. it is functionally limited to low stress levels and monotonic loading, especially with respect to the confinement because gas is used as the confining medium, the volume to be compressed is large, the confining pressure is controlled by a manual valve, the pressure vessel is a thin wall tube, and the membrane attachment detail allows two free membrane surface areas to expand and contract causing large fluctuations in the size of the volume required for pressurization and depressurization, which, in turn eliminates efficient dynamic response regardless of the confining medium being used;
2. there is no deformation measurement system, so the device can only be used for strength testing which only requires a load measuring device.
A triaxial cell similar to the Texas triaxial type is the Hveem stabilometer illustrated in Standard Test Methods for Resistance to Deformation and Cohesion of Bituminous Mixtures by Means of Hveem Apparatus by ASTM in ASTM D 1560 (Nov. 27, 1981) and Mechanics, Operation, Calibration, and Diaphragm Installation of the Stabilometer by California DOT in CalTrans 102 (1978). The Hveem stabilometer improves upon the Texas triaxial cell in the area of the membrane attachment detail, wherein it effectively reduces the free membrane surface area.
The Hveem stabilometer includes the following disadvantages:
1. the pressure is applied to the membrane using a manual crank, which eliminates the possibility of practical dynamic response;
2. a fluid is used as the confining medium, but the measurements are extremely sensitive to entrapped air, and the casting and assembled components of the cell make it difficult to purge air from the system for calibration purposes;
3. the device does not measure engineering properties which independently characterize the material behavior, it measures a test property which can only be used for comparing materials tested in the same apparatus or must be correlated with engineering property measurements to indirectly infer properties of materials which have not been tested for true engineering properties.
The instant invention teaches a material testing machine comprising dual test spaces, one for unpressurized specimen testing and one for pressurized axisymmetric triaxial testing. The pressurizable test space is adapted to accept a triaxial testing module, said triaxial testing module either comprising sealing components that enable the pressure vessel to emulate a standard geotechnical type pressure vessel, or a rapid operating triaxial cell module comprising sealing components and a membrane which may be optionally instrumented for axial and/or radial strain measurement.
It is an objective of the instant invention to provide a material testing machine which obviates the disadvantages of prior machines by being compact in size, thereby enabling it to be used in crowded laboratories or for mobile applications.
It is another objective of the instant invention to provide a material testing machine which has a dual test space, one of which can be configured for either standard geotechnical triaxial testing or for rapid triaxial testing using an integral reusable membrane and a membrane-mounted displacement measurement system configurable for axial, radial, or combined radial and axial measurements.
It is still another objective of the instant invention to provide a material testing machine which can be made relatively economically, and which can be easily operated, said testing machine comprising a loading reaction frame that is also a pressure vessel over a portion of its length thus eliminating the need for a separate loading frame and additional triaxial test fixture that must be mounted in the frame, and which further is constructed and arranged to have an open test space over the remaining portion of its length configurable for use with additional separate test fixtures commonly used in the prior art.
Other objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention. The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof.